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Durability of compressed
earthen bricks stabilised with
guar gum for sustainable
construction
By
Yann d’Offay – Mancienne
The Year of 2020
A dissertation submitted in fulfilment of the requirements for the
degree of Master of Engineering in Civil Engineering
Newcastle University
School of Civil Engineering and Geosciences
Newcastle University
CEG8099 Coursework 1
School of Civil Engineering & Geosciences
DISSERTATION MARK SHEET
Module Details
CEG8099 – Investigative Research Project
Student
Yann d’Offay - Mancienne
Supervisor
Agostino Bruno
Second Marker
Sadegh Nadimi
Dissertation
Durability of compressed earthen bricks stabilised with guar gum for
sustainable construction
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Abstract (or Summary), and
Introduction
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Literature Review
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Aim & Objectives
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Methodology
20 (15 – 25)
*
Results
20 (15 – 25)
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Discussion
25 (20 – 30)
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Conclusions &
Recommendations
15
15
Project Management
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5
5
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2
Abstract
In this dissertaion an investigation into the durability properties of guar gum stabilised
compressed earth blocks in comparison to those stabilised with cement and a control of
unstabilised blocks. The soil used was Nagen soil from Toulouse in France. The stabilised
blocks were composed of 4% and 8% cement, 2% and 4% guar gum. A total of 15 blocks were
produced and cured for 7 days, 2 of each type used for a drip erosion test and the remaining five
for an immersion test. Results from the drip erosion test show that the guar gum blocks
outperformed all other blocks as the 2% and 4% guar blocks having an average depth of
penetration of 8.045mm and 5.690mm. For comparison, the 4% and 8% cement stabilised
blocks experienced average depths of penetration of 12.495mm and 10.86mm with the
unstabilised blocks averaging 19.375mm. These results were supported by visual inspection of
the blocks used in the immersion test and it was concluded that the reason for these results may
be a combination of three explanations. Firstly, was the guar gum forming a hydrophobic
surface around soil particles that deflects water, secondly, that the low density of the guar gum
results in a lower void ratio and hence water absorption than the same mass of cement, and an
accelerated curing time for blocks stabilised by guar gum.
3
Contents
Abstract ......................................................................................................................................... 3
List of figures and tables ............................................................................................................... 6
Acknowledgements ....................................................................................................................... 6
Abreviations .................................................................................................................................. 6
Introduction ................................................................................................................................... 7
Literature review ........................................................................................................................... 9
Project objectives and goals ...................................................................................................... 9
Project overview ................................................................................................................... 9
Research objectives ............................................................................................................... 9
Literature review ..................................................................................................................... 10
A brief history of earthen construction ............................................................................... 10
Modern earth construction: The compressed stabilised earth block ................................... 10
Advantages & Disadvantages of CSEB’s ........................................................................... 11
Changes required to improve CSEB’s significantly ........................................................... 13
Engineering properties of CSEB’s ...................................................................................... 13
Stabilisation......................................................................................................................... 22
Conclusion .......................................................................................................................... 26
MATERIALS AND METHODS ................................................................................................ 27
Material characterisation ......................................................................................................... 27
Soil properties ..................................................................................................................... 27
Grain size distribution ......................................................................................................... 27
Plasticity.............................................................................................................................. 28
Specific gravity ................................................................................................................... 28
Compaction procedure ............................................................................................................ 28
4
Manufacturing procedure ........................................................................................................ 30
Testing procedures .................................................................................................................. 35
Drip test............................................................................................................................... 35
Immersion test ..................................................................................................................... 36
Results ......................................................................................................................................... 37
Drip test................................................................................................................................... 37
Immersion test......................................................................................................................... 39
Discussion ................................................................................................................................... 40
Conclusion .................................................................................................................................. 41
Recommendations for future experimentation ............................................................................ 42
References ................................................................................................................................... 42
Appendix ..................................................................................................................................... 47
Project management statement................................................................................................ 47
Impact of COVID-19 on research objectives .......................................................................... 49
5
List of figures and tables
Figure 1 Compaction curves and variation of compressive strength with water content and
compaction pressure (Olivier and Mesbah, 1986) ...................................................................... 18
Figure 2 Variation of compressive strength with dry density (Morel et al., 2007) ..................... 19
Figure 3 Grain size distribution curve(Bruno et al., 2016) ......................................................... 27
Figure 4 Plasticity properties of the tested soil in relation to the admissible region for
compressed earth bricks(Bruno et al., 2016)............................................................................... 28
Figure 5 Mould assembly diagram.............................................................................................. 29
Figure 6 Mould assembly format with soil added....................................................................... 30
Figure 7 Representation of drip test ............................................................................................ 36
Figure 8 Photograph after drip test ............................................................................................. 37
Figure 9 The comparison between depth of penetration(mm) and the weight change(g) of the
blocks after testing ...................................................................................................................... 39
Figure 10 Photograph after immersion test ................................................................................. 39
Table 1 Soil mix proportions for the drip test blocks.................................................................. 30
Table 3 Weight of soil mix (excluding self- weight of the plastic bag) used in the drip test...... 31
Table 4 Weight of soil mix (excluding the self-weight of the plastic bag) used in the immersion
test ............................................................................................................................................... 31
Table 5 Maximum load achieved when moulding blocks for the drip test ................................. 32
Table 6 Maximum load achieved when moulding the blocks for the immersion test................. 32
Table 7 Block dimensions before curing for the drip test x=length, y=width, z=height(mm).... 33
Table 8 Block dimensions after curing for the drip test x=length, y=width, z=height(mm) ...... 33
Table 9 Block dimensions before curing for the immersion test x=length, y=width,
z=height(mm).............................................................................................................................. 34
Table 10 Block mass before and after equalisation for the drip test ........................................... 34
Table 12 Block mass before curing for the immersion test ........................................................ 35
Table 13 Results of depth of penetration for drip test ................................................................. 38
Table 14 Block mass after testing for the drip test ..................................................................... 38
Acknowledgements
Abreviations
AS1289 : Australian Standards ................................................................................................... 13
ASTM standard D559-89 : American Society for Testing and Materials D559-89 ................... 13
BS 3921 : British standard 3921 ............................................................................................12, 13
CEB : Compressed earth block ..................................................................................................... 8
CSEB : Compressed stabilised earth block ................................................................................... 7
6
IRS : Initial rate of suction .......................................................................................................... 12
IS1725 : Indian Standard ............................................................................................................ 16
LL : Liquid limit ......................................................................................................................... 12
NF P94-051 : Normes Françaises P94-051 ................................................................................. 12
NZS standard : New Zealand Standards ..................................................................................... 16
OPC : Ordinary Portland Cement ............................................................................................... 19
PAB : Pressed adobe block ......................................................................................................... 11
PI : Plasticity index ..................................................................................................................... 12
PL : Plastic limit.......................................................................................................................... 12
SBR : Styrene Butadiene Rubber ................................................................................................ 20
SLS1282 : Sri Lanka Standards 1282 ......................................................................................... 16
UNE 41410 : Spanish Association for Standardization .............................................................. 16
Introduction
Compressed stabilised earth blocks otherwise known as CSEB’s have been used in the
construction industry for many years and if this category is expanded to include adobe or earth
construction the amount of time can be measured in millennia. From ancient Egypt to the
Romans, Native Americans and so on the earth has always been used as the base for most
construction materials. A CSEB usually consists of a mixture of soil, water and a stabiliser; this
means they cure without having to go through a kiln or by using large amounts of cement to
create concrete blocks and thus have been shown to be incredibly environmentally friendly in
terms of overall emissions and embodied energy in comparison to concrete blocks and kiln fired
bricks. A stabiliser in the context of construction is a chemical additive used to improve certain
qualities of a building material, in this case soil. For the subgrades of roads, bitumen is usually
commonplace and in wet environments where access is difficult or to strengthen road
embankments, lime is often used. For CSEB’s, cement is used as it drastically increases the
compressive strength of the soil mixture when moulded into a block, but also its durability
properties; mostly with regards to water damage from the environment. Unfortunately, the
adoption of cement in CSEB’s still presents one significant limiter, the environmental benefits
of using CSEB’s are bottle-necked due to cement being a toxic material that comes from nonrenewable sources and requiring substantial amounts of energy to produce whilst releasing
carbon into the atmosphere. Also, cement stabilised blocks have not maximised their built
environment management properties, i.e. acoustic and thermal performance, despite
substantially outperforming any other kind of block. This is where bio-stabilisers become
important. A bio-stabiliser in the context of the construction industry and for simplicity is a
chemical additive used to enhance the qualities of a building material whilst originating from a
completely natural and renewable source. Chitosan, xanthan gum and guar gum are a few of the
7
bio-stabilisers currently under investigation. The advantage of bio-stabilisers comes from the
fact that as they are naturally sourced and renewable, they present significant environmental
benefits over cement in terms of embodied energy, thermal qualities, carbon sequestration and
possibly economics. However, bio-stabilisers do come with their uncertainties. It is well
documented that xanthan gum and guar gum stabilised blocks are only slightly weaker than
cement stabilised blocks with regards to compressive strength. What is less well understood is
how well these bio-stabilisers compare with respect to durability, as there is some notable
speculation as to how they react to environmental aggravators, most notably water damage. It is
for this reason that further experimentation into the durability properties of bio-stabilised earth
blocks must conducted, so as to better inform the construction industry of their viability in
construction and possible contribution to the reduction of damage to the environment caused by
human construction. In this paper, guar gum, which is extracted from guar seeds and used
mainly in food stabilisation; was selected for testing as it is a more readily available biostabiliser that requires no prior preparation and can be bought at low cost in bulk. Blocks made
of completely unstabilised soil, cement stabilised soil and guar gum stabilised soil will be
produced through the use of a mould and loading press for the purpose of experimentation. A
drip erosion test, which involves inclining a block and dripping water on it from a height will be
conducted and the results from this experiment are to be recorded through measurements and
images. An immersion test which, as the name suggests, involves immersing a block in water
for a certain amount of time will also be conducted, with results taking the form of only images
due to the disruption caused by COVID-19 making it impossible to attain more scientific and
empirical measurements. Results from DVS tests were also greatly affected by the disruption.
The full material characterisation, methodology and interpretation of the results are included in
this paper.
8
Literature review
Project objectives and goals
Project overview
The construction industry depends heavily on the production of bricks, and in particular, earth
fired bricks. On a planetary scale, civil works and building construction consumes 60% of the
raw materials extracted from the lithosphere with building representing 24% of these global
extractions (Zabalza Bribián et al., 2011). With the sustainability of the construction industry
coming into focus within recent years, research into the field of stabilised earth bricks has been
made paramount to reducing the levels of embodied energy in the manufacture of these crucial
structural elements. As of late, the traditional stabilisers used in compressed stabilised earth
blocks have been scrutinised for their environmental and economic impacts, most notably
Ordinary Portland cement and lime. These impacts have galvanised research into more
environmentally friendly alternatives such as bio-stabilisers. In this paper, the various forms of
stabilisers and their processes are examined with the physical properties and proportions of the
soils most ideal for stabilisation. Laboratory experimentation will be used to compare the
compressive strength, and durability characteristics of guar gum stabilised earth blocks to those
stabilised by cement.
Research objectives
The objectives of this paper are to analyse and compare the mechanical and durability properties
of compressed stabilised earth blocks that have been stabilised with various concentrations of
cement and guar gum and compare them against each other and an unstabilised set of control
blocks. The cement stabilised blocks are being tested to form a good comparison to those
stabilised by guar gum whilst a set of unstabilised blocks will be produced and tested in order to
form the control for both stabilisers to be compared to. The reasoning behind this is the
availability of both cement and guar make them more comparable for the purposes of this study.
The main objectives are outlined as follows:

To manufacture three sets of earth block made of Nagen soil from France and stabilise
them with cement, guar gum and to have one set act as a control and hence remain
unstabilised. The blocks will be manufactured using the compaction method.

The blocks will be cured for 7 days

The cement stabilised blocks will contain concentrations of cement of 4% and 8%; the
guar stabilised blocks will contain concentrations of guar of 2% and 4%.

Manufacture an additional two blocks of each type to be used in the drip testing as per
UNE 41410 standards.

Manufacture an additional block of each type to be used in immersion testing.
9

Record all relevant results, for the drip testing it will be the depth of penetration and
change in weight whereas for the immersion testing it will be the ratio between the
leftover filter material and the initial mass of the block.

Perform DVS tests.
Literature review
The following section will give a brief introduction into the use of earthen materials in
construction over human history and review the existing literature on the idealised
mechanical and durability properties of soils which are fit for stabilisation. Also, the
mechanisms through which several commonly used stabilisers operate are examined with
the inclusion of research into current bio-stabiliser candidates. Furthermore, the various
tests available to determine those properties are investigated so as to guide any further
research.
A brief history of earthen construction
The use of earthen materials in construction is no recent occurrence and have been used for
millennia as they required only readily available materials such as soil and water. The ancient
Egyptians made good use of this material, and the remnants of their history from 2500BC show
the procedures they used to manufacture earthen blocks (Niroumand et al., 2013). Since the time
of the Egyptians, earthen building technique spread throughout the middle east and into Africa,
and then into Europe. Even in North America structures were being constructed through the use
of earthen materials which demonstrates how widespread this technique was across the planet.
Modern earth construction: The compressed stabilised earth block
A CSEB is a compressed stabilised earth block; hence, it is a brick or a structurally
corresponding element that has been made entirely from a soil mixture, usually of sand and fine
materials with the addition of a stabiliser. Abdullah et al., (2017) outlined a method for
preparing CSEB's. Soil physical testing should initially be conducted so as to make certain the
suitability of a laterite soil for CSEB production. The two methods of testing most frequently
conducted are grain size analysis test and Atterberg limits test. The process starts by preparing
the raw materials, most notably the Portland cement, sand, laterite soil and water. Particle sizes
must range between 2 mm for sand and less than 5 mm for the laterite soil so as to make certain
of the binding between all materials when mixed. The three different mix proportion ratios of
cement: sand: laterite soil employed in this study were 1:1:9, 1:2:8, 1:3:7. The water added must
not contribute more than 15% by the ratio. The CSEB mixture was then poured into a mould
and compacted using a hydraulic compaction machine. The CSEB’s were covered and protected
from sunlight and rainfall so as to prevent them draining quickly during the curing process for
two different periods of 7 days and 28 days. The most common form of stabiliser is Portland
cement as it is cheap and easily accessible in most countries, whereby lime is also used but is
10
slightly more challenging to come by, especially in developing countries. Bitumen emulsions
are used to stabilise soils usually in the subgrades of roads and the various methods and theories
behind soil stabilisers will be discussed further.
Advantages & Disadvantages of CSEB’s
Since the inception of civilization, earth has been used as a building material in the construction
of structures used for a multitude of functions, but most commonly dwellings. Easton, (1996)
states that rammed earth construction is a cheap way of providing shelter since earth is an
abundant resource with Frescura, (1981) adding that earth has excellent cultural and
architectural importance in addition to its mechanical, thermal and environmental benefits.
Morton, (2007) stated that earth blocks might be used as a suitable replacement for conventional
blocks so long as they are not expected to be subjected to severe climatic conditions. This
means that despite their fundamental advantages, earthen blocks still suffer from crippling
disadvantages, both of which will be discussed below.
Advantages:

Earth construction is economically beneficial: The required raw materials are found
almost anywhere on the planet and have been used in some of the poorest countries to
provide low-cost housing options (Lal, 1996, Minke, 2006).

It requires simple tools and less skilled labour: Guillaud et al., (1995) notes that
where the construction norms significantly depend on the use of masonry elements
(fired bricks, stone and sand-cement blocks), the compressed earth block is more readily
adopted and forms an additional building resource serving the socio-economic
development of the building sector. Furthermore, the wide range of presses and
production units available on the current market makes the material very flexible to use.
As production ranges from small-scale to medium and large-scale semi-industrial or
industrial, CEB’s and CSEB’s are useful in rural and urban contexts and can meet very
widely differing needs, means and objectives which is a point also supported by
Kateregga, (1983).

Reduction of embodied energy: Deboucha and Hashim, (2011) stated that extraction,
transportation and manufacture of earthen materials require only 1% of the energy
needed for the production of cement-based materials with Little and Morton, (2001)
adding that the manufacture of earth blocks only need 33% of the energy required to
manufacture conventional fired bricks of similar dimensions, more specifically 440
kWh/m3 in comparison to 1300 kWh/m3.

Reduction of operational energy: Earthen material can absorb vapour from humid
environments and release them into dry ones, resulting in changes in weight of about 35% (Cagnon et al., 2014). This can, therefore, help to regulate hygroscopic conditions
inside a building by absorbing, storing and releasing moisture as necessary, which is a
11
very advantageous property that can contribute to ensuring healthy levels of ambient
humidity inside dwellings while reducing air conditioning needs (Bruno et al., 2016).

Acoustic insulation: Raw earth provides significant noise reduction properties due to
their high dry density which contributes to an increased noise reduction index, allowing
for a more acoustically sound environment between the internal and external walls of a
building (Hadjri et al., Bruno et al., 2016).

Recycling or safe disposal of demolition waste: Bossink and Brouwers, (1996) stated
that the building and the construction industry cause a significant part of waste
generation. Lower deposition costs and lower purchasing costs of fresh materials from
reduced waste generation provides a significant benefit to construction companies. In
their study the waste generation during several Dutch residential construction projects
was quantified and analysed in detail, concluding that about 1-10% by weight of the
purchased construction materials, depending on the material, leaves the site as waste.
Despite these considerable advantages, CSEB’s still have significant drawbacks related to
soil selection and durability, as will be discussed below.
Disadvantages:

Inadequacy of local soil selection: Most studies agree that earthen construction
requires a baseline mix of at least 70% coarse material or otherwise sand and a 30%
fine material content, usually in the form of clayey-silt. The clay plays a very
important role as it is responsible for the capillary bonding of coarse grains, which
is the primary contributor of strength in unstabilised earth (Jaquin et al., 2009).
Additionally, the clay fraction has a significant interaction with the atmosphere by
absorbing, storing and releasing moisture depending on ambient humidity, thus
contributing to the hygro-thermal regulation of indoor space (Bruno et al., 2016). If
the local material does not contain the right proportion of constituents, then the soil
may very well need to be transported from further away, thus negating its beneficial
energy consumption and financial cost aspects. A workaround for this would be to
use stabilisers to alter the properties of the material, preferably from a sustainable
and ecologically friendly source.

Poor quality control: Two levels of quality control have been identified, a
"precautionary" level which consists of monitoring the selection, mixing and
storage of soil constituents before compaction. This level can be performed on-site
with relative ease whereby the "confirmatory" level is more concerned with
ensuring that the final density, strength and durability of the built product are
compliant with design requirements. This level is difficult to conduct on-site due to
the variability in material characteristics due to workmanship (Crowley, 1997,
Bruno et al., 2016).
12

Susceptibility to moisture ingress: Capillary action (capillary effect, or wicking)
is the ability of a liquid to flow in narrow spaces without the assistance of, or even
in opposition to, external forces like gravity. In earthen materials, this effect causes
the rapid absorption of any water the material comes into contact with.
Experimentation has shown that during the initial phase of exposure of a stabilised
raw earth sample to free water, the moisture content increases linearly with the
square root of time, resulting in the phenomenon often referred to as the "wick
effect" (Washburn, 1921). This moisture ingress severely reduces the earthen
materials mechanical properties, whereby in a construction context results in limited
structural reliability. However, Hall and Djerbib, (2004) showed that controlling
volume, pore size and degree of saturation of the material could help in limiting
moisture ingress.

Unreliable durability: Ancient earthen structures that have been preserved for
thousands of years owe their longevity to the arid or dry climates they often inhabit.
Problems with durability arise in wetter climates whereby the rainfall causes surface
erosion and moisture ingress, particularly in the case of unstabilised earth
structures. An experiment by Bui et al., (2009) measured between 5 mm and 10 mm
of erosion from the surface of a 400 mm thick unstabilised earth wall exposed to a
wet continental climate over twenty years. These results were synonymous with
those from a test done by the Massachusetts Institute of Technology in 2005 which
showed a surface erosion of about 5-7 mm over nine years of exposure to the
temperate climate of the North-eastern coast of United States (Dahmen, 2015).
Stabilisation with cement or lime has shown to increase durability and resistance to
water damage. However, no biological stabiliser has shown to provide the same
level of protection as they tend to either decompose or suffer from organism attack.
Changes required to improve CSEB’s significantly
Compressed stabilised earth blocks have shown to have similar compressive strengths
as regular concrete blocks and are reasonably durable if stabilised with cement or lime.
However, use of those stabilisers somewhat defeats their environmental benefits and
hence naturally occurring stabilisers which are environmentally friendly, cheap, easily
accessible and most importantly durable in the context of water damage are paramount
to the adoption of CSEB's around the world.
Engineering properties of CSEB’s
Grain size distribution
Grain size distribution has shown to be an important property when forming an ideal soil for
stabilisation and one of the most common methods of determining it is through sieve analysis.
13
Due to its ease of interpretation, simplicity and low cost, the use of this technique is very
common for many measurements. Methods may be simple shaking of the sample in sieves until
the amount retained becomes more or less constant. Alternatively, the sample may be washed
through with a non-reacting liquid (usually water) or blown through with air current. Other
methods of determining grain size distribution include air elutriation analysis, photo analysis
and various sedimentation techniques.
M. Carmen Jime ́nez Delgado, (2007) analysed 20 technical documents relating to the use of unstabilised soils in earthen construction in order to provide a comparison of approaches. They
found that particle size distribution is one of the most emphasized properties due to its
significant effect on soil behaviour, noting that the inclusion of small particle sizes is highly
valuable for CEB's. A similar study was done by Vasilios Maniatidis, (2003) whereby over 200
pieces of scientific literature relating to rammed earth construction were examined and found
that soils having significant quantities of fine particles such as clay and silt, as well as large
quantities of sand are ideal for earthen construction. Further evidence supporting these
conclusions is provided by a study performed by Wu et al., (2013), whereby the authors
concluded that the compressive strength of adobe blocks increases parabolically with the
increase clay-silt content. The methodology of the test consisted of laying out adobe blocks in
the ratio of 1:1 (soil: sand) and soil mortars (soil in the context of this test meaning a mixture of
clay and silt) in the ratios of 1:0.8, 1:1, and 1:1.2 (soil: sand) respectively. The compressive
strengths of the block and soil mortar were both determined on the uniaxial compressive test of
single adobe block. The tests were carried out on a 200 kN capacity electro-hydraulic machine
and conducted at a constant loading rate of 12 kN/min (2.56 MPa/min) to failure in about 1 min.
A 100 × 100 × 25 mm steel plate was connected to the upper-end support to distribute axial
loads evenly. The impact of the ratio between clay and silt was not examined, and hence further
research into this relationship is required. Kouakou and Morel, (2009) tested a soil containing
25% clay of diameter < 2 μm which were made into unstabilised adobe and pressed adobe
blocks (PAB's) that were subsequently subjected to unconfined compression tests. This paper
further reinforces the theory that a significant quantity of clay content in earthen blocks
contributes greatly to its compressive strength noting an average compressive strength of 34.5Mpa. The authors added that despite these increases in compressive strength, the blocks were
then left susceptible to damage due to water saturation, hence still requiring stabilisation to
prevent this. The collective conclusion of these authors seems to align with the tests done in a
paper by Muguda et al., (2019) whereby the geotechnical properties of a recycled soil mixture
such as particle size gradation, Atterberg limits and linear shrinkage were compared with an
unamended soil mixture to assess the changes due to recycling. The paper idealised one of the
characteristics of a prospecting soil for CSEB's as being composed of 70% sand and 30% fine
material, contributing to a plasticity limit of below 20%.
14
The effect of grain size distribution on the hydraulic behaviour of soil cannot be underestimated.
A study done by Jaquin et al., (2008b) involved tests carried out on rammed earth to determine
their water retention properties. The authors used a soil of (0.25:0.60:0.15; aggregate:sand: clay)
whereby two mixes (A&B) were created having 10% increase in sand and a 10% increase in
clay respectively. The dry density/water content relationship for the basic mix was obtained
using the vibrating hammer compaction test and showed an optimum water content of
approximately 8–10%, achieving dry densities of between 2017 and 2061 kg/m3. Mix A came
out having a lower water content than mix B, indicating that the proportion of clay within a soil
mix is crucial to its hygroscopic behaviour. A separate test done by Hall and Djerbib, (2004)
used a modification of the BS 3921 IRS test apparatus called the 'wick test' on earthen bricks
composed of a variety of soil samples. This was done to understand the correlation between
particle size distribution and moisture ingress in rammed earth structures. The results drew a
similar conclusion to that of Jaquin et al., (2008b), in that particle size distribution is crucial in
determining the rate at which moisture ingress may occur due to capillary suction.
Plasticity
The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its
plasticity index, plastic limit, and liquid limit.
Liquid limit: The liquid limit (LL) is the water content at which the behaviour of clayey soil
changes from the plastic state to the liquid state. However, the transition from plastic to liquid
behaviour is gradual over a range of water contents, and the shear strength of the soil is not zero
at the liquid limit. The precise definition of the liquid limit is based on standard test procedures
such as the Casagrande method whereby the moisture content at which it takes 25 drops of the
cup to cause the groove to close over 12.7 millimetres is defined as the liquid limit.
Plastic limit: Plastic Limit is the water content at the change from a plastic to a semisolid state.
This is determined by rolling threads of moist soil that break apart at a diameter of 3 mm
according to the norm NF P94-051
Plasticity index: The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity
index is the size of the range of water contents where the soil exhibits plastic properties. The PI
is the difference between the liquid limit and the plastic limit (PI = LL-PL).
The Atterberg limits of a particular soil play a significant role in its successful stabilisation. A
paper by Walker, (1995) set out to assess the influence of soil characteristics and cement content
on the physical properties of stabilised soil blocks found that drying shrinkage of the blocks is
governed by the plasticity index of the constituent soil. The results concluded that the drying
shrinkage requirements of earth blocks are met when then the plasticity index is <20. The
method the author used was to dry mix clay soil and river sand (by volume) in varying
proportions between 100% clay: 0% sand and 15% clay: 85% sand. In addition to the modified
15
soils, blocks were produced using three natural soils (denoted soil B, E and M). These
additional tests were undertaken to check the general applicability of the modified soil test
results. Ordinary Portland cement was used throughout for chemical stabilisation. For each
modified soil mixture cement was added in proportions of 1: 10, 1: 15 and 1: 20 (cement: soil
by dry volume). The blocks were then cured for 28 days, whereby the average drying shrinkage
of three blocks from each mix was determined in accordance with BS 6073. Shrinkage
movements were assessed using a 200 mm demec gauge. The blocks were initially prepared by
immersing them in a tank of water at 23°C for 96 hours. After removal from the water, initial
demec readings were taken. Each block was then placed in a ventilated oven set at 50 °C. The
demec gauges were periodically recorded, for approximately 14 days, until constant readings
were attained. Drying shrinkage was determined by subtracting the final dry from the initial
saturated demec strain gauge readings. Determination of the resistance to water erosion was
undertaken using the wire brush test specified in ASTM standard D559-89. Burroughs, (2008)
conducted an experiment whereby 104 different soils were used in stabilisation experiments
taken from 29 rammed earth construction sites across Australia. Using 40kg of soil excavated
from trial pits or by hand up to 3m deep, the 104 soils were tested with combinations of the
stabilisers lime and cement. A variety of tests to determine the grain size distribution, liquid
limit, plastic limit, plastic index and linear shrinkage were performed in accordance with
AS1289. The objective of the experiment was to relate value ranges of natural soil properties
such as plasticity, texture, and shrinkage to the degree of the predisposition of soils to
stabilisation for rammed earth wall construction. The study concluded that soils with a plasticity
index of 15 containing 64% sand had a 90% stabilisation success rate and were hence ideal for
use as a stabilised soil.
Dry density
Dry density refers to the density of the soil when it is taken in the dry state. The soil mass is
commonly a mixture of air, water and soil solids. The dry density refers to the soil solids. It is
otherwise called the dry unit weight of soil. The dry density should be calculated using the
expression as below,
𝛾𝑑 =
𝑀𝑠
𝑉𝑇
Here, γd is dry density, Ms is the dry mass of the soil solids, and VT is total soil volume.
Another expression for calculating dry density while doing the compaction test is as below,
𝛾𝑑 =
𝛾
𝑤
1 + 100
16
Here, the term γd is the unit weight of the soil in a dry state, γ is bulk unit weight, and w is
moisture content expressed in per cent.
The Proctor compaction test is a laboratory method of experimentally determining the optimal
moisture content at which a given soil type will become most dense and achieve its maximum
dry density. These laboratory tests generally consist of compacting soil at known moisture
content into a cylindrical mould with a collar of standard dimensions of height and diameter
using a compaction effort of controlled magnitude. The soil is usually compacted into the mould
to a certain number of equal layers, each receiving a number of blows from a standard weighted
hammer at a specified height. This process is then repeated for various moisture contents, and
the dry densities are determined for each. The graphical relationship of the dry density to
moisture content is then plotted to establish the compaction curve. The maximum dry density is
finally obtained from the peak point of the compaction curve and its corresponding moisture
content, also known as the optimal moisture content.
Effect of dry density on mechanical properties
Olivier and Mesbah, (1986) studied the effect of compaction pressure on the mechanical
properties of the “Isle d’Abeau” earth (50% sand, 33% silt and 17% kaolinitic clay). Cylindrical
samples with a diameter of 11 cm and a unitary aspect ratio were produced by static double
compaction at different pressure levels from 1.2 MPa to 10 MPa. For each compaction pressure,
they tested differing water contents in order to determine the optimum water content and
corresponding maximum dry density. Upon observation, it was noted that compaction curves
shift towards lower values of water content as the compaction stress increases (Figure 1). After
compaction, samples were stored at constant temperature (27 °C) and relative humidity (60%)
and then tested under unconfined compression until failure. Results confirmed that compressive
strength increases as dry density increases.
17
Figure 1 Compaction curves and variation of compressive strength with water content and
compaction pressure (Olivier and Mesbah, 1986)
Another study by Morel et al., (2007) investigated the mechanical characteristics of compressed
earth blocks. It showed that strength strongly increases with increasing dry density (Figure 2)
for both unstabilised and stabilised earth. This is true regardless of whether non-swelling “twolayer” clays (e.g. kaolin) or swelling “three-layer” clays (e.g. bentonite) are prevalent in the fine
fraction of the earthen material. Clay minerals are generally classified into three-layer types
based upon the number and arrangement of tetrahedral and octahedral sheets in their basic
structure. These are further separated into five groups that differ with respect to their net charge
(Barton, 2002).
18
Figure 2 Variation of compressive strength with dry density (Morel et al., 2007)
Durability
Earth blocks are particularly susceptible to water damage; hence, an examination of the factors
that most greatly influence its durability must be performed; such as its moisture buffering
capacity. Determining the correct tests to perform is paramount to producing a more informative
and usable set of results to draw conclusions from. Papers have been written assessing the
relevance of several methods of testing either by performing the tests themselves or by
reviewing the literature surrounding them. A paper by Cid-Falceto et al., (2012) set out to assess
the durability of compressed stabilised earth blocks against rain. They used three different kinds
of blocks, i.e. unstabilised, 6% cement and 8% cement-lime. Two main tests were performed
with variations between them, the spray erosion test whereby three variants of the test were
used. The first complying with the NZS standard (NZS, 1998, Zealand, 1998), the second with
the SLS1282 standard and the third with the IS1725 standard (BIS, 1982). This first set of tests
concluded that the spray erosion test in itself is only suitable for use with the stabilised blocks
but is deemed too aggressive for the unstabilised block. Also, it recommended that the spray
erosion test using the SLS1282 standard was the most applicable due to its methodology and
system of evaluation. The drip erosion test was done according to the UNE 41410 standard,
which involves two samples of CSEB; stating that if the hole produced is equal to or less than
10mm the block is of suitable quality. The authors concluded that the drip erosion test is
suitable for unstabilised blocks but not for those that have been stabilised as no quantifiable
differences are produced. To make the test applicable, its criteria of evaluation should relate to
the loss of weight of the block and not the depth of erosion. Also, the spray and drip erosion
19
tests were conducted on three different faces of the block, concluding that that varying the faces
used in the spray erosion has no real effect on the erosion results. The authors further concluded
that earth blocks stabilised with cement and lime are suitable for construction. Obonyo et al.,
(2010) set out to establish the optimal stabilisation strategy for earth bricks while focussing on
those that can be used to counter the deterioration caused by wind-driven erosion. The authors
produced bricks stabilised by cement, lime and fibre, which were compared to factory produced
bricks. The bricks were cured through exposure to sunlight for 2-3 weeks with water being
sprinkled over them to optimise the process while a protective sheet was laid over them to
reduce the risk of damage to them from the environment. After curing, the external faces of the
bricks had 0.1m of their surface exposed to the spray of a pressure washer positioned 0.5m from
their surfaces. Usually, for the Bulletin 5 test, a pressure of 40-70MPa is used, but the authors
restricted it to 2.07MPa and 4.14MPa in order to assess the resilience of the engineered brick.
The bricks were sprayed for one hour, with readings for the depth of erosion taken every 15
minutes. The results showed that the factory produced bricks showed little to no deterioration,
while the cement and lime stabilised bricks showed a maximum of 25mm of erosion. The fibre
stabilised bricks performed the most poorly with a maximum depth of erosion, showing some
correlation with Cid-Falceto et al., (2012) in the context that cement and lime are still at the top
of the hierarchy when it comes to durability to rainwater based erosion. The authors did state
that the bricks were produced in a laboratory environment and that those produced in the field
may very well behave differently, with the lime fraction being susceptible to chemical
deterioration. Additionally, a review of the literature surrounding durability tests for stabilised
earth blocks was done by Ogunye and Boussabaine, (2002) and aimed to form a generalised
framework that can be used in the methodologies for testing the weather-based durability of
these types of blocks. Their conclusions correlated in some degree with Cid-Falceto et al.,
(2012) in that current methods of determining durability are not entirely accurate. However,
their opinions diverge as Ogunye and Boussabaine, (2002) were adamant that the current
methods, particularly those which involve accelerated weathering are impractical and give way
to significant inconsistencies with those done in the field, and hence must be disregarded
entirely. They suggested that long term erosion tests should instead be conducted in the field as
they give more reliable results as compared to accelerated weathering tests. These papers
highlight the importance of selecting the appropriate test when it comes to durability and
suggests that some degree of modification to them may be required in order to form relevant
conclusions. Because guar gum is a more sustainable bio-stabiliser than either cement or lime,
in this paper, the durability of guar gum stabilised earth blocks will be assessed using the drip
erosion test as per the UNE 41410 standards but with addition of the measurement of the loss of
weight of the block. Comparing the measurement of the depth of penetration and loss of weight
would comply with UNE 41410 standard as well include the criterion of evaluation proposed by
20
Cid-Falceto et al., (2012). Also, a standard immersion test will be conducted in order to further
reinforce conclusions drawn from the drip tests.
Tests used to assess blocks durability include:
Wearing test: According to ASTM D559-03 (ASTM, 2003), earthen blocks are immersed in
water for 2 minutes, removed and then dried at 105 °C for 24 hours. After drying, eighteen wire
brush strokes of about 13 N force are applied to each side of the block and four strokes to each
end of the block. The wearing performance of an earthen block is determined as the dry mass
loss after 12 wetting – drying cycles.
Spray erosion test: According to NZS 4298 (Committee, 1998), this test consists in spraying
the face of an earthen sample with a jet of water for one hour and measuring the resulting
erosion depth. The jet of water is applied at a constant pressure of 0.05 MPa from a nozzle
placed 0.47 m from the sample. The exposed area of the specimen corresponds to a circle of 150
mm in diameter. Earthen blocks are then classified in five different classes depending on
erosion depth. Other standards propose different procedures that vary for the duration of the test,
pressure of the water jet, distance of spraying, exposed area and erosion tolerance criteria.
Drip test: According to the standards UNE 41410 (Cid-Falceto et al., 2012), this test consists in
releasing 500 ml of water in 10 minutes from a height of 1 m on a sample that is inclined of 27°
respect of the horizontal. If erosion is not deeper than 10 mm, the brick is suitable for
construction.
Immersion test: This test allows a first qualitative assessment of the material durability.
Earthen samples are weighed before testing and then dipped in water for 10 minutes. The mass
loss is determined by filtering the residual material from water. It is then dried at 40 °C for 24
hours, left at the atmosphere and weighed. Material loss is determined as the ratio between the
mass of the filtered material and initial mass of the sample.
Contact test: This test reproduces the application of a mortar joint or a coating on earthen
bricks. For this purpose, an absorbent cloth (cellulose) is dipped in water and then placed on the
visible face of the brick. The applied amount of water must correspond to 0.5 g/cm2. Samples
are then stored for 24 hours in a sealed container on a rack above the water. Then, the absorbent
cloth is removed, and bricks are exposed to atmospheric conditions for two days. After this, an
examination of the bricks is performed to detect cracks and/or permanent deformations owed to
swelling.
Suction test: This test determines the response of earthen blocks when exposed to a temporary
excess supply of water. For the suction test, three earth block halves are equalised under
standard hygro-thermal conditions (T = 23±2 °C; RH = 50±5%) until constant mass. Then, fired
bricks are placed in a pan forming a continuous plane. The pan is then filled with water up to 121
5 mm below the upper edge of the fired brick. A layer of an absorbent cloth is laid on top of the
fired bricks. Earthen blocks are subsequently placed on the absorbent cloth, thus starting the
suction test. During testing, water is adsorbed by the earthen blocks, and extra water must be
added to keep the same level inside the pan. Samples are visually assessed at 30 min, 3h and
24h after the beginning of the test to detect cracks and permanent deformations owed to
swelling.
Stabilisation
Soil stabilisation a general term for any physical, chemical, mechanical, biological or
combined method of changing a natural soil to meet an engineering purpose which
usually results in increased compressive or tensile strength and durability. Common
stabilisers include Portland cement, lime and bitumen; all of which have uniquely
stabilised soil but share the same problem, negatively affecting the environment through
either embodied energy or the toxicity of their chemical components. Biological
stabilisers, otherwise known as bio-stabilisers, are those that originate from natural
sources tend and to be more environmentally friendly will be examined in detail with
the other forms of stabilisation in the section below.
Cement stabilisation
The reaction mechanism for a cement-based binder is mostly hydraulic, meaning it
requires only water in order to begin improving the mechanical properties of the
mixture it has been added to; a procedure referred to as curing. In terms of mechanical
performance cement usually outperforms all other stabilisers; however, its
environmental and durability properties which are closely linked to its production
methods are those for which the former is poor and the latter decent. It is estimated that
7% of the anthropogenic CO2 corresponds to the Portland cement (OPC) production
(Escalante-Garcia et al., 2009). Cement is also chemically vulnerable to sulphate or
chemical waste attack from certain ground conditions making its use in structural
foundations a cause for concern (Tomlinson and Boorman, 2001).
Lime stabilisation
Clay soil can be stabilised via the addition of a small percentage, by weight, of lime,
that is, it enhances many of the engineering properties of the soil such as shear,
compressive and tensile strength. This produces improved construction materials, and
so the technique has been used notably in highway, railroad and airport construction to
improve subgrades and bases. Generally, the amount of lime stabiliser needed to modify
a clay soil varies from 1-3%, but optimal values for compressive strength lie between 512% (Guettala et al., 2002). When lime is added to clay soils, calcium ions are
combined initially with or adsorbed by clay minerals which lead to an improvement in
soil workability, that is, to an increase in the plastic limit of the clay and generally to a
decrease in its liquid limit (Bell, 1989). In order to control swelling and shrinkage, lime
22
stabilisation techniques are generally adopted for clayey soils. When the lime is
thoroughly mixed with the soil, the clay minerals present in the soil react with the lime.
These lime–clay reactions lead to the formation of a water-insoluble gel of silicate and
silicate–aluminates, and (with time) this gel finally crystallises into hydrates of calcium
silicate, calcium aluminates. The cementitious gel formed coats the soil particles and
establishes bonds. The pace of lime–clay reactions and formation of cementitious gel is
slow as it conventionally takes 28 days for them to develop their strength fully. These
reactions take place continuously, even in the presence of just a little moisture in the
surrounding atmosphere (Hall et al., 2012).
Foamed bitumen stabilisation
Foamed bitumen (also known as foamed asphalt, foam bitumen or expanded asphalt) is
a mixture of air, water and bitumen. When injected with a small quantity of cold water,
the hot bitumen expands and forms a fine foam. This expanded bitumen mist is
incorporated into the mixing drum where the bitumen droplets are attracted to and coat
the finer particles of pavement material, thus forming a mastic that effectively binds the
mixture together. The foamed bitumen is usually used in the stabilisation of soils for
road construction in particular but comes with inherent disadvantages such as being
more expensive than lime and requiring bespoke equipment to manufacture (Kendall et
al., 1999).
Bio-polymers
Stabilisation using biopolymers is achieved through “hydrogels” which are formed
through the interaction of soil, biopolymer and water particles. Unlike cementitious
bonds formed due to hydration of cement, these “hydrogels” bind soil particles through
a combination of chemical bonds and soil suction (Reddy et al., 2019). Hydrogels have
become very popular due to their unique properties such as high-water content, softness,
flexibility and biocompatibility. Natural and synthetic hydrophilic polymers can be
physically or chemically cross-linked in order to produce hydrogels (Caló and
Khutoryanskiy, 2015). Hydrogels are three-dimensional, hydrophilic, polymeric
networks capable of absorbing large amounts of water. The networks are composed of
homopolymers or copolymers and are insoluble due to the presence of chemical
crosslinks (tie-points, junctions), or physical crosslinks, such as entanglements or
crystallites. The latter provides the network structure and physical integrity. These
hydrogels exhibit a thermodynamic compatibility with water which allows them to
swell in aqueous media (Peppas et al., 2000).
Compared to Portland cement or lime, polymers tend to be expensive. Despite this,
there are multitudes of polymers available for soil stabilisation such as Urea
23
Formaldehyde Rejoin (UFR) and Styrene Butadiene Rubber (SBR) (Ahmed, 2019).
Polymers can be mixed with soil in the form of a liquid in order to fill the pores and
harden the soil structure. According to Hall et al., (2012), in order for polymeric
stabilisation to be used, the following requirements must be met:

the polymer must have the ability to adhere to soil particles with the

the assistance of water (adhesive)

it must be internally cohesive

it must be capable of working sufficiently (polymerising) at high humidity

and at low/non-elevated ambient temperatures

it must be miscible with water to produce a low viscosity liquid.
Current literature on bio-polymer stabilisation
Bio-polymers are those that have been produced by living things and contain monomeric units
in covalent bonds that form larger structures. There are two main classes of bio-polymer, those
called “bio-replacement” polymers which are made from and enhanced by sugar, lignin and
cellulose. The second class called “bio-advantaged” polymers which are made of minimally
modified vegetable oils, proteins, and some other biological monomers. Since they are naturally
occurring, it has been theorised that there may be environmental benefits of using bio-polymers
instead of traditional stabilisers for the production of CSEB’s (Guo, 2014).
A study done by Aguilar et al., (2016) investigated the feasibility of using chitosan biopolymer
as an admixture, or as an external coating, for earthen constructions to improve their resistance
to water-induced degradation and key mechanical properties. The resistance to water-induced
degradation was determined via contact angle and drip erosion tests. In the context of that study,
the contact angle (which was determined using a high-resolution camera) was defined as the
angle formed by the liquid from the sessile drop and the surface of the material, with angles less
than 90° denoting a high wettability while those with an angle greater than 90° denoting a low
wettability. The drip erosion test in this study entailed placing a block on a surface inclined at
an angle of 27° to the horizontal. The block was then subjected to water drops released from a
point exactly 1m above the centre of the block. The drops were then released at a rate of 50 mL
per minute. The influence on mechanical properties was measured through compressive, tensile
and three-point bending tests. The results showed that applying a 0.5% coating solution was
successful in protecting earthen materials from water erosion. Materials with an admixture of 13% were even shown to give earthen materials a high-water erosion resistance while
simultaneously improving said materials mechanical properties.
24
Another bio-polymer that has been studied is Carrageenan, which is a substance extracted from
red and purple seaweeds. A study by Nakamatsu et al., (2017) evaluated it as a bio additive to
improve the physical and mechanical properties of adobe constructions. In this study, the sessile
drop technique and drip erosion test were performed to evaluate changes in water permeability.
The results showed that there was a positive effect on both the tensile strength and water erosion
resistance capability of earthen materials despite exposure to sunlight did lead to some sample
degradation, but this did not completely negate the positive effect of the additive.
Additional research by Villamizar et al., (2012) set out to investigate the effect of the addition of
coal-ash and cassava peels on the engineering properties of compressed earth blocks. A series of
test compressed earth blocks were made using a clay-rich soil, without coal-ash and stabilised
with coal-ash, in a Cinva-Ram hydraulic machine. The samples were tested for flexion,
compression and absorption in order to observe their performance. Results show that the
compressive and bending tests reveal that the compressed earth blocks stabilised with coal-ash
produced the best results using a dose less than or equal to 5%. However, doses greater than 5%
generate more flexible and fragile compressed earth blocks. Adding cassava peels to the clayed
soil increases the required water content for extrusion (apparent plasticity) but doing so for
construction was advised against as the cassava peels were susceptible to organism attack.
Research done by Muguda et al., (2017) investigated two gums (guar and xanthan) in their use
as soil stabilisers. Guar and xanthan gum, the latter made from the fermentation of sugar by a
specialised bacterium and the former from guar beans were shown to increase the mechanical
properties of earthen materials significantly through unconfined compressive tests. Guar gum is
a neutrally charged polysaccharide with large hydroxyl groups (Chudzikowski, 1971), and
forms a network of hydrogels between soil particles and free water via hydrogen bonds (Chen et
al., 2013). Xanthan gum is an anionic polysaccharide (Katzbauer, 1998, Garcıa-Ochoa et al.,
2000). It may interact with cations of the clay portion of a soil mix to form chemically stronger
ionic bonds in addition to hydrogen bonds (Chang et al., 2015). All the samples were statically
compacted to achieve the initial dry density of 19.62 kN/m3 having a porosity of 16.98% and
pore void volume of 14.63 cm3. The samples were then left to cure by drying to the laboratory
atmosphere (relative humidity of 50% and a temperature of 210°C) and were then tested after 7
and 28 days. Finally, water content and total suction (using a WP4C Potentiometer) were
measured on portions of the remains. For comparison, identical tests were carried out on
samples of the unamended soil after seven days of air curing and on samples stabilised with
8.0% cement by mass after 7 and 28 days of air curing. Similarly, for the biopolymer-stabilised
samples, air curing took place inside the laboratory atmosphere. The guar and xanthan mixtures
were tested 1.5-2% concentrations, both of which showing strength improvements comparable
to that of cement stabilisation despite the xanthan samples showing some strength reductions at
the end of 28 days. In terms of tensile strength, the xanthan samples far outperformed the guar
25
samples and were shown to be 38% stronger in tension than the cement stabilised samples at 28
days. A study conducted by Latifi et al., (2016) tested xanthan gum mixed with soil using
similar testing methods showed significant correlation with these conclusions. Chang et al.,
(2016) noted that during production these biopolymers sequester carbon dioxide, the opposite of
which is true for cement, whereby large amounts of greenhouse gases are released. Despite this,
Lo et al., (1997) found that the energy required for the production of xanthan gum can be
greater than that required for an equivalent amount of cement.
All of these studies provide some indication of the level of performance of bio-stabilised blocks,
yet as most of them were done using the unmodified drip erosion test which has been noted as
being somewhat ineffective at giving qualitative results, hence there is a gap in the research.
This gap can be addressed by performing the drip erosion test with a different evaluation of
criteria, one based around the loss of weight of the block after being subjected to the drip
erosion test. Also, none of the studies included an immersion test and hence the inclusion of this
test would help form a more rounded comprehension of the durability properties of earth blocks
stabilised with bio-polymers. Finally, the amount of research done to quantify the actual
durability of guar gum stabilised earthen materials is very limited, with most studies focusing on
the compressive strength of the final material and hence additional research into the durability
of materials stabilised by guar gum would further fill this gap in current research.
Conclusion
In conclusion, compressed stabilised earth blocks hold many notable advantages over regular
concrete blocks such as being more economically viable and having a reduced embodied energy
content despite having disadvantages such as the difficulty of implanting proper quality control
in their manufacture and being susceptible to moisture ingress. Also, it is evident that the grain
size distribution is important in selecting soils appropriate for stabilisation as the relevant
literature suggests a soil mixture of 30% clay and silt and 70% sand. The examined literature
also stated results that showed a plasticity index of 15 would give the largest success rate for
stabilisation and that the effect of dry density cannot be disregarded when it comes to the
strength of the stabilised block. Durability is arguably the most important factor affecting the
adoptability of compressed stabilised earth blocks, hence selecting the appropriate tests is
crucial to measuring this property. Spray erosion testing has been noted as the best for testing
stabilised blocks with drip testing being noted as not providing conclusive results unless its
criteria of evaluation are changed to the amount by which its weight changes. Long term testing
in the field was argued as the best gauge of durability, but for the purposes of practicality are
not applicable to this paper. The mechanisms of bio-stabilisation revolve around the formation
of hydrogels, and the polymers involved are either bio-replacement or bio-advantaged polymers.
Current literature has reviewed bio-stabiliser candidates such as chitosan, carrageenan, a coal
ash and cassava peel combination, xanthan and guar gum. Both xanthan and guar show
promising results as their performance was virtually on par with that of concrete, which is why
26
it is evident that further research in the field of bio-polymer stabilisation is required as they are
an environmentally and economically more viable alternative for soil stabilisation. Xanthan and
guar gum in particular should be investigated concerning their water resistance and durability
properties as they are both readily available in the market and are only required in low
concentrations, effectively making them an economically sound product. As they perform well
in compression tests, these materials would have a bright future if they were not so liable to
degradation to the environment. Hence experimentation using the appropriate techniques into
their durability is paramount to bringing them one step closer to becoming the norm in
environmentally and economically sustainable construction.
MATERIALS AND METHODS
Material characterisation
Soil properties
The soil is known as “Nagen” soil and originates from Toulouse in France and was provided by
a brick factor of the same name.
Grain size distribution
Nagen soil contains a higher proportion of fine particles than most soils which according to
Jaquin et al., (2008a), which allows it to hold more water than soils with a higher proportion of
coarse materials, resulting in a stronger hygroscopic behaviour. The grain size distribution was
determined through a combination of sedimentation and wet sieving according to the norms XP
P94-041 and NF P 94-057.
Figure 3 Grain size distribution curve(Bruno et al., 2016)
27
Plasticity
The properties of plasticity of the fines portion of the soil (particles<400µm) were measured as
per NF P 94-051. The fines portion of the Nagen soil is classified as inorganic clay of medium
plasticity in accordance with the Unified Soil Classification System (USCS ASTM D2487-06).
Figure 4 Plasticity properties of the tested soil in relation to the admissible region for
compressed earth bricks(Bruno et al., 2016)
Specific gravity
The specific gravity of the Nagen soil was determined through the use of a pycnometer test with
accordance to NFP 94-054 via the calculation of the average of three measurements and is equal
to 2.65.
Clay activity
Clay activity is the ratio of plasticity index to clay fraction(fraction of particles<2µm) and was
determined to be 0.79, classifying the clay fraction as being normally active (Skempton, 1953).
This is indicative of the soil being composed of primarily illitic material with illite being a three
layer clay with a limited swelling capacity and good bonding qualities, suiting it particularly to
raw earth construction (Dierks and Ziegert, 2002).
Compaction procedure
To compact the soil into blocks, a mould must be used into which the soil is packed into and
eventually compacted by the loading press. The mould can be described as consisting of four
separated parts, with two of each part type being parallel to one another. The parts which are the
longest are labelled 1 and 2, and those which are the shortest are labelled 3 and 4. The parts are
28
fitted together and held in place by four bolts and are made of high resistance steel so as to resist
the large lateral pressures exerted by the soil during compaction. The soil is poured into mould
and an aluminium block that has an indentation line on it is placed on top of the soil. The
indentation line denotes at what pressure should cease to be exerted and compaction should
stop. A steel disc is then placed on top of the block to evenly distribute the load. The mould
does not contain any mechanism that would allow for water to drain. Furthermore, the soil is
compacted close to the optimum and hence any drainage of water would not be expected.
Figure 5 Mould assembly diagram
The mould assembly procedure is depicted in figure 6 and is as follows:
1. Parts 3 and 4 are assembled so as to fit within the indentations of parts 1 and 2.
2. A steel plate is added in between the assembled sections and pushed to the bottom of
the assembly. This is to provide a surface onto which the soil mix can be poured so that
the soil mix does not fall through the bottom of the mould.
3. Four steel bolts, nuts and spacers are used to stabilise and tighten the connections
between the entire mould assembly.
4. The soil mixture is then poured into the mould and the aluminium block is placed on
top of it and gently hammered with a leather hammer so as to create a level surface and
snug fit.
5. A steel disc is then placed on top of the steel block in order to evenly distribute the load
from the press.
6. The mould is then placed inside the press and brought up to within a 5mm of the
compaction surface.
7. A compaction pressure is applied by the press until the steel block has sunk into the
mould and its indentation line is at the same level as the top of the mould’s surface.
8. The mould is then removed from the loading press, disassembled carefully by hand and
the block is subsequently removed from the mould.
29
Figure 6 Mould assembly format with soil added
Manufacturing procedure
As per the testing requirements a total of 15 blocks were manufactured so as to perform both the
drip and immersion tests. As the tests were to be performed on blocks that only had 7 days to
cure, the production schedule was adjusted accordingly so that it took a total of 10 days to
manufacture a set of blocks fit for experimentation. The procedure was as follows:
1. Day 1: Bake the soil in an oven at 108 degrees Celsius for a minimum of 24 hours.
2. Day 2: Mix the soil in the following proportions for both the drip test and immersion
test.
Table 1 Soil mix proportions for the drip test blocks
Drip test
Block type
Soil
Cement
Guar
Volume
Quantity
mass(g)
mass(g)
gum
of
for drip &
mass(g)
water(ml)
immersion
test
Unstabilised
2000
0
0
160
2&1
4% cement
1920
80
0
160
2&1
8% cement
1840
160
0
160
2&1
2% guar
1960
0
40
160
2&1
1920
0
80
160
2&1
gum
4% guar
gum
30
Once the constituents had been mixed thoroughly by hand the water was added and
further hand mixing was carried out. Following this the constituents were mixed in an
industrial mixer for 2. 5 minutes at low speed before being removed, being hand mixed
for a further 2 minutes and then put back into the industrial mixer for 2.5 minutes. Once
a batch was completed the soil mix was put in a sealed plastic bag, labelled
appropriately, weighed (so as to quantify how much soil mass was lost during mixing)
and then left to rest for 24 hours. There are some differences in the final weights of the
bags and that could be attributed to both losses in the fine powder content of the soil
during the mixing process and the water being mixed in when the soil was still hot from
the oven, which thus lead to evaporation and some loss of mass, as can be seen in tables
3 and 4.
Table 2 Weight of soil mix (excluding self- weight of the plastic bag) used in the drip test
Drip test
Block type
Bag weight(g)
Unstabilised (1)
2124.59
Unstabilised (2)
2143.79
4% cement (1)
2144.44
4% cement (2)
2150.44
8% cement (1)
2148.54
8% cement (2)
2154.79
2% guar gum (1)
2150.99
2% guar gum (2)
2152.64
4% guar gum (1)
2149.04
4% guar gum (2)
2152.69
Table 3 Weight of soil mix (excluding the self-weight of the plastic bag) used in the immersion
test
Immersion test
Block type
Bag weight(g)
Unstabilised (1)
2116.64
4% cement (1)
2135.84
8% cement (1)
2137.89
2% guar gum (1)
2153.84
4% guar gum (1)
2145.60
3. Day 3: On this day the soil mixes were moulded into blocks by a loading press. The
procedure was to assemble a steel mould with specific dimensions, pour in the soil mix,
31
place a steel block on top followed by a steel disc and then proceeding to pressing the
blocks. Once the blocks had been pressed to the desired proportions, the load was held
momentarily and then released gradually. The load required to press the blocks, their
dimensions and final weight were recorded and are shown below in table 5. The cause
for the increase in load required for compaction can be attributed to the void ratio of the
blocks, as with the guar gum which has a large volume, much of the voids within the
block are filled thus causing the block to require a large compaction force. The weight
of the blocks was important to record as they would decrease in weight over their curing
period due to moisture loss. The bricks were stores in an area where the temperature
was 20℃ (± 3℃) and the humidity at 45% (± 5%).
4. Day 10: The dimensions and weight of the blocks were recorded again and the testing
was conducted. Due to the disruption caused by COVID-19 the measurements of weight
and dimensions after curing for the immersion test blocks were not recorded.
Table 4 Maximum load achieved when moulding blocks for the drip test
Drip test
Block type
Load (KN)
Unstabilised (1)
120
Unstabilised (2)
145
4% cement (1)
155
4% cement (2)
140
8% cement (1)
110
8% cement (2)
175
2% guar gum (1)
133
2% guar gum (2)
127
4% guar gum (1)
185
4% guar gum (2)
185
Table 5 Maximum load achieved when moulding the blocks for the immersion test
Immersion test
Block type
Load(kN)
Unstabilised (1)
130
4% cement (1)
175
8% cement (1)
198
2% guar gum (1)
160
4% guar gum (1)
197
32
Table 6 Block dimensions before curing for the drip test x=length, y=width, z=height(mm)
Drip test
Dimensions(mm) [before curing]
Block type
x1
x2
y1
y2
z1
z2
Unstabilised
200.49
200.57
100.46
100.29
51.62
51.91
200.52
200.61
100.46
100.53
50.9
50.88
4% cement (1)
200.59
200.61
100.48
100.52
51.88
51.55
4% cement (2)
200.47
200.52
100.36
100.46
51.85
51.79
8% cement (1)
200.52
200.56
100.43
100.35
53.77
53.43
8% cement (2)
200.49
200.55
100.56
100.49
52.16
51.63
2% guar gum
200.68
200.73
100.5
100.51
52.21
52.32
200.62
200.68
100.41
100.38
52.46
52.4
200.67
200.78
100.47
100.48
52.33
51.93
200.67
200.8
100.44
100.43
51.78
51.83
(1)
Unstabilised
(2)
(1)
2% guar gum
(2)
4% guar gum
(1)
4% guar gum
(2)
Table 7 Block dimensions after curing for the drip test x=length, y=width, z=height(mm)
Drip test
Dimensions(mm) [after curing]
Block type
x1
x2
y1
y2
z1
z2
Unstabilised
200.14
200.08
100.13
100.35
51.17
50.82
199.95
199.98
100.21
100.11
50.51
5.59
4% cement (1)
200.13
200.03
100.17
100.1
51.3
51.69
4% cement (2)
200.11
199.95
100.12
100.03
51.63
51.44
8% cement (1)
200
200.02
100.1
99.99
53.13
53.55
8% cement (2)
200.02
200.06
100.16
100.1
51.38
51.75
2% guar gum
199.94
200.16
100.07
100.1
52.03
52.09
199.92
199.96
100.12
100.07
52.26
52.07
(1)
Unstabilised
(2)
(1)
2% guar gum
(2)
33
4% guar gum
199.94
200.2
100.13
100.2
51.83
51.66
199.92
200.09
100.15
100.14
51.45
51.33
(1)
4% guar gum
(2)
Table 8 Block dimensions before curing for the immersion test x=length, y=width,
z=height(mm)
Immersion
Dimensions(mm) [before curing]
test
Block type
x1
x2
y1
y2
z1
z2
200.68
100.47
100.51
50.96
50.93
200.44
200.52
100.56
100.4
50.79
51.07
200.47
200.54
100.5
100.43
50.64
50.58
200.54
200.63
100.47
100.41
51.3
51.37
200.61
200.69
100.49
100.5
51.93
51.95
Unstabilised 200.69
(1)
4% cement
(1)
8% cement
(1)
2% guar
gum (1)
4% guar
gum (1)
Table 9 Block mass before and after equalisation for the drip test
Drip test
Block type
Mass before equalisation (g)
Mass after
equalisation (g)
Unstabilised (1)
2117.3
2060.20
Unstabilised (2)
2138.55
2077.20
4% cement (1)
2137.3
2079.20
4% cement (2)
2145.45
2086.45
8% cement (1)
2137.9
2075.75
8% cement (2)
2146.85
2085.20
2% guar gum (1)
2146.4
2083.00
2% guar gum (2)
2147.8
2075.20
4% guar gum (1)
2144.55
2084.90
4% guar gum (2)
2149.05
2077.85
34
Table 10 Block mass before curing for the immersion test
Immersion test
Block type
Mass before equalisation (g)
Unstabilised (1)
2114.65
4% cement (1)
2133.75
8% cement (1)
2136.45
2% guar gum (1)
2152.4
4% guar gum (1)
2145.05
Testing procedures
Drip test
The procedure for the drip test performed followed the UNE41410 standard which is
summarised by:
1. Positioning the sample (block) on a surface that is inclined at 27⁰ to the horizontal. This
was done using an adjustable testing box.
2. At 1m above the centre of the block, a container filled with 500ml of water must be
positioned. The water in the container must be allowed to drip onto the sample at a rate
of 50ml/min. This was done using a modified testing cylinder with an adjustable tap to
control flow. The tap was set to allow for a flow rate of 50ml/min.
3. Allow the water to drip onto the sample for 10min.
4. Remove the sample and record its weight.
5. Measure and record the depth of penetration produced in the sample. This was done
using a Vernier calliper. If the depth of penetration is in excess of 10mm then the
sample has failed to meet the criteria required to pass the drip test. Only two of each
kind of block are required for the test.
35
Figure 7 Representation of drip test
Immersion test
The immersion test involves immersing a block in water for 10 minutes and calculating the
ratio between the mass of filter material left over and the initial mass of the block. This ratio
is then used as the criteria for evaluation of the test. The procedure is as follows:
1. Fully immerse block in water that is contained and start a timer for 10 minutes.
2. Once the time is up, remove the block from the water.
3. Remove the filter material from the container.
4. Allow dry for 24 hours at a temperature of 40℃ and measure the weight of the filter
material and the block to calculate the ratio.
36
Results
Drip test
Figure 8 Photograph after drip test
37
Table 11 Results of depth of penetration for drip test
Drip test
Block type
Depth of penetration(mm)
Unstabilised (1)
18.88
Unstabilised (2)
19.87
4% cement (1)
12.07
4% cement (2)
12.92
8% cement (1)
10.84
8% cement (2)
10.88
2% guar gum (1)
7.63
2% guar gum (2)
8.46
4% guar gum (1)
5.97
4% guar gum (2)
5.41
Table 12 Block mass after testing for the drip test
Drip test
Block type
Unstabilised (1)
Unstabilised (2)
4% cement (1)
4% cement (2)
8% cement (1)
8% cement (2)
2% guar gum (1)
2% guar gum (2)
4% guar gum (1)
4% guar gum (2)
Mass (g) [after testing]
2136.25
2131.65
2213.75
2235.65
2223.6
2269.35
2146.8
2134.95
2135.05
2125.6
38
Figure 9 The comparison between depth of penetration (mm) and the weight change (g) of the
blocks after testing
Immersion test
Figure 10 Photograph after immersion test
From the table 13 it is clear that the unstabilised blocks performed the poorest having their
average depth of penetration of 19.375mm being significantly higher than any of the other
samples. Upon visual inspection of the samples seen in figure 8 they were seen to have almost
partially disintegrated under the action of the water droplets with much of their initial soil mass
converting into a thick mud. The blocks containing 4% cement performed significantly better
yet still having failed the criteria for passing by having an average depth of penetration of
12.495mm. However, they also showed some significant structural degradation, with this being
39
true for the blocks containing 8% cement having an average depth of penetration of 10.86mm.
The blocks containing 2% and 4% guar gum by far outperformed the other blocks by having
average depths of penetration of 8.045mm and 5.690mm respectively. Visually, these blocks
also experienced minimal to no degradation. To form an idea of what may be causing these
differences, a graph plotting the comparison between depth of penetration and change in weight
of the blocks after testing was plotted in figure 9. It is assumed that a larger portion of the
change in weight may be attributed to water absorption. For the unstabilised blocks it can be
seen that they only absorbed a small amount of water before becoming saturated and
experiencing physical degradation; being used as controls, this comes as no surprise. The
comparison between the cement stabilised blocks and those stabilised by guar gum show the
most interesting results. The cement stabilised blocks absorbed much more water and
experienced much more degradation than those stabilised by guar gum. These results are further
substantiated by those from the immersion test seen in figure 10 which shows how the
unstabilised blocks has almost completely dissolved, with 4% and 8% cement blocks suffering
less significant but similar degradation. The guar gum blocks are shown to be the only blocks to
retain most of their original shape, indicating that they are more hydrophobic than the other
blocks or resistant to water damage; with this possibly contributing to their performance.
Discussion
Before any conclusions are drawn it must re-stated that the blocks were tested after being left to
cure for a period of 7 days and as such consideration must be taken in the interpretation of these
results. Firstly, it is evident that the guar gum stabilised blocks have far outperformed their
cement and unstabilised counterparts. There are three possible main reasons for this which may
be mutually independent of each other. The first reason may be that the addition of guar gum
into the soil mix and eventually the block may create a hydrophobic surface around the soil
particles which contribute to a soil matrix that reacts to water ingress by deflecting instead of
absorbing and holding it in until saturation; possibly due to the network hydrogels formed
between the soil particles (Chen et al., 2013), thus leading to a much lower water saturation
level, indicated by the graph in figure 3. The second reason for this result may be that the curing
time until maximum durability for the guar gum stabilised blocks might be shorter than that of
the cement blocks, meaning that as the blocks were only cured for 7 days the guar gum
stabilised blocks would have reached a point closer to their practical maximum durability than
the cement stabilised blocks. The final reason could be that as the guar gum has a lower density
than the cement (0.80-1.00g/cm3 for guar gum and 1.44g/cm3 for cement) that a larger volume
of the guar gum is required to be mixed in with the soil in order to create a mix having the
required dry mass of 2000g. This could mean that more of the voids in the soil structure are
being filled by guar gum than could be possible by cement, hence the final soil mix has a much
smaller void ratio. This means that there are less voids available for water absorption, hence less
water is absorbed which results in a lower pore pressure causing the blocks to stay more intact
40
under the action of the droplets and water absorption. Unexpectedly, modifying the drip tests as
stated by Cid-Falceto et al., (2012) was not required as the stabilised blocks showed visual
degradation; however this may have been due to the curing period only having been 7 days.
These results are very promising and coincide with the visual inspection of the blocks used in
the immersion tests. Due to the disruption caused by COVID-19, numerical results were unable
to be recorded, however, the physical degradation of the blocks immersed in water bring forth
similar conclusions about the durability of guar gum stabilised blocks.
Conclusion
In this paper, an investigation into the durability properties of guar gum stabilised compressed
earth blocks compared to cement stabilised compressed earth blocks was conducted. The
literature surrounding the topic was examined in order to provide the necessary empirical and
experimental information required to justify the use of guar gum over any other bio-stabiliser
and the method of testing required to perform. A modified drip erosion test was initially
selected for the first round of testing as the literature had suggested that the stabilised
compressed earth blocks would not show any appreciable degradation under the action of the
water droplets. This method involved setting the criteria of evaluation of the drip erosion test to
being the total change in mass of the block instead of solely the depth of penetration caused by
the water droplets. Experimentation showed the direct opposite of this assumption as visible
physical degradation was immediately apparent following the end of the drip erosion test.
Fifteen blocks in total were produced in order to test whether guar gum stabilised compressed
earth blocks could outperform those stabilised by cement in terms of durability. The block
compositions were selected as per recommendations from the literature and consist of
unstabilised blocks, 4% and 8% cement, 2% and 4% guar gum with 160ml of water added to
each soil mix. The blocks were moulded through the use of a loading press and were allowed to
cure for a period of 7 days before testing commenced. For the drip erosion test, two of each
block type was used. The blocks were placed at 27⁰ to the horizontal and had 500ml of water
drip onto them for a period of 10 minutes (50ml/min) from a height of 1m above their centre.
The results showed that the guar gum blocks far outperformed the cement and unstabilised
blocks in terms of both visual physical degradation and more importantly depth of penetration;
being the only set of the blocks to have met the criteria of evaluation for passing the drip
erosion test. Three main reasons were speculated for this result. Firstly, the guar gum may have
created a hydrophobic surface surrounding the soil particles and thus causing deflection to be
the main mechanism for preventing damage caused by water ingress. Secondly, the guar gum
could possibly take less time to cure i.e. the block reaches maximum strength and durability
much faster than cement, hence at the 7 day mark the cement was not as far along in the curing
process as the guar gum thus contributing to their poorer performance. Thirdly, since guar gum
41
is less dense than cement, it requires a greater volume to be added to the soil mix for the desired
dry mass to be achieved, hence more voids within the soil structure are filled leading to a
decreased volume of voids available for water absorption. For the immersion tests, one block of
each type was produced. The test involves submerging a block in water for a period of 10
minutes and then allowing them to dry at 40℃ for 24 hours. The ratio of filter material to initial
mass is then used as the criteria for evaluation. Due to the disruption caused by COVID-19, the
necessary measurements required to determine the ratio of filter material to initial mass was
impossible to calculate, however, visual inspection of the blocks immediately after conducting
the immersion test showed the guar gum blocks had suffered significantly less physical
degradation than the other blocks, thus outperforming them. Although crude, this observation
coincides with those from the drip erosion test and points to a conclusion that in terms of
durability with regards to water damage, compressed earth blocks stabilised with guar gum
outperform those stabilised with cement and may be suitable for construction. The results from
this research could significantly enhance the sustainability of the construction industry by
contributing to the literature surrounding the reliability of guar gum stabilised compressed earth
blocks for use within the construction industry.
Recommendations for future experimentation
For future experimentation, several recommendations can be made to further explore the
reliability and usability of bio-stabilised earth blocks.
1. Conducting drip erosion and immersion tests on blocks that have had 7 and 28 days to
cure.
2. To conduct experiments that compare guar gum stabilised compressed earth blocks to
those stabilised with other bio stabiliser such as xanthan gum and chitosan. This is to
provide a broader set of experimental results and conclusion in order to justify their use
in the construction industry.
3. To perform experimentation that looks into the economic and environmental
sustainability of guar gum in the context of the construction industry.
4. Performing spray erosion tests on bio-stabilised earth blocks.
5. Testing the effects of chemical attack on bio-stabilised earth blocks.
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Appendix
Project management statement
Date
Agenda
Steps
24/02/2
Bake soil
Set oven to 108◦C
020
Bake soil for 24hrs
25/02/2
Mix cement and guar with
020
soil
Produce brick soil mixture
2 X 0% cement
2 X 4% cement
2 X 8% cement
2 X 2% guar
2 X 4% guar
Add 160ml of water
Mix for 2.5mins at speed 1
Remove and hand mix
Mix for another 2.5mins on speed 1
Place soil mix in plastic bag
Leave overnight - 24hrs
26/02/2
Compress soil mixes into
020
blocks
Assemble steel mould components
Remove base off press
Put mould on press plate
Put soil in mould
Compress soil
Remove compressed blocks
Weigh blocks and record
Measure dimensions and record
Leave to cure for 7 days
47
04/03/2
Perform drip test
Weigh blocks and record
Perform unconfined
Measure dimensions and record
020
compression test
Fill 500ml bottle with water
Set drip to 1m above block
Angle block at 27◦to the horizontal
Test for 10 mins
Measure depth of penetration and record
Weigh blocks and record
Set compressor on 1mm/min
Compress the blocks till failure and place them in
plastic bags
Take small sample of broken blocks
Place in tin and weigh
Place in oven for 24hrs
05/03/2
Weigh samples
Remove samples from oven
020
Weigh samples and record
09/03/2
Mix cement and guar with
020
soil
Produce brick soil mixture
1 X 0% cement
1 X 4% cement
1 X 8% cement
1 X 2% guar
1 X 4% guar
Add 160ml of water
Mix for 2.5mins at speed 1
Remove and hand mix
Mix for another 2.5mins on speed 1
Place soil mix in plastic bag
Leave overnight - 24hrs
10/03/2
Compress soil mixes into
020
blocks
Assemble steel mould components
48
Remove base off press
Put mould on press plate
Put soil in mould
Compress soil
Remove compressed blocks
Weigh blocks and record
Measure dimensions and record
Leave to cure for 7 days
17/03/2
Perform immersion tests
Weigh blocks and record
Perform unconfined
Fully immerse blocks in water for 10mins
020
compression tests
Collect residual filter material
Weigh blocks and residual filter material
Place blocks and residual filter material in oven at
40◦C for 24hrs
18/03/2
020
Weigh material
Weigh residual filter material and block and
record
Impact of COVID-19 on research objectives

Unable to obtain full results from immersion test, this includes the ratio of mass
between filter material and the original mass of the block.

Could not support conclusion with results from the DVS tests.
49
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